This invention relates to forming reinforcing components, and in particular to forming a composite fibre reinforced plastics article to conform with a contoured surface.
Articles formed in accordance with the present invention have a variety of applications, but by way of example only, and without limitation to the generality, reference will be made to the forming of reinforcing components used in the aircraft industry, for example aircraft stringer sections.
When forming such components, in order to maximise their structural effect, it is desirable to ensure that the fibres of the component follow the contours thereof as closely as possible. Such a component may be produced by manually laminating a succession of prepreg plies so as to form a multi-layered stack, but this is very time consuming. xe2x80x9cPrepregxe2x80x9d material consists of one or more plies of Jibson""s reinforcing material pre-impregnated with uncured resin or like matrix material.
1. Description of the Prior Art
It is known to form a thick stack of uncured prepreg material to provide reinforcement in one direction. As can be seen from FIG. 1 of the accompanying drawings, a flat sheet 2 of uncured prepreg having a large plurality of carbon fibres embedded in epoxy resin, with the fibres aligned in the direction of the arrow A, has been formed into a 90xc2x0 bend over the edge of a forming tool 4. In this way, an L-shaped stiffener is made, and it can be envisaged that with a larger sheet of prepreg two bends could be made so as to provide a channel section, with the fibres thereof extending undirectionally. Such a method of forming a component, however, has the disadvantage that it cannot be utilized when the surface to which the article has to conform is contoured, for example containing a joggle or a ramp, or being undulating. Such geometric features introduce diversions into the path of the fibres, which, instead of following the contour, flow around the bend radius (see FIG. 2), and are thus no longer properly aligned in the required load-bearing direction. The reinforcing effect is thus not so efficient.
Alternatively, cutting a shaped blank such that it achieves good conformity to the underlying contoured surface results in load bearing fibres being cut. FIG. 2 of the accompanying drawings shows a forming tool 6 that is a modification of the tool 4 of FIG. 1, in that its upper surface is provided with a ramp 8. FIG. 3 shows a rectangular blank 10 that has been subjected to two cuts along trimlines 12 and 14 so as to achieve the conformity not only with the 90xc2x0 bend but also along the ramp 8 of FIG. 2.
2. Summary of the Invention
It is one object of the present invention to provide an improved forming method.
In accordance with one aspect of the present invention there is provided a method of forming a substantially planar, multi-layer, composite article that comprises a plastics material reinforced with fibres that extend substantially continuously in one direction in the plane of each layer, wherein a majority of the layers are aligned in a main direction, wherein the article is disposed over a contoured surface of a forming tool, wherein the article is constrained against movement in a direction substantially perpendicular to the plane of said main direction layers, wherein force is applied to the article in the plane thereof in a direction substantially perpendicular to the said main direction of said fibres, thereby to urge the article into conformity with the contoured surface of the tool whilst substantially maintaining the continuity of the said fibres.
Depending on the viscosity of the resin of the composite article, heat may need to be applied thereto in order to ensure that the layers, or laminates, are able to slide with respect to each other. Typically, although the fibres within each layer will be substantially unidirectionally aligned, not all the layers will be aligned. The actual alignment of the plurality of layers will depend on the structural load requirements, of tension and torsion for example. Normally a majority of the fibres, and of the layers are aligned in one main direction for reinforcement, referred to as the 0xc2x0 direction. Various other layers may then be aligned at 45xc2x0, 190xc2x0, and 135xc2x0 thereto as required. For example, 50% of the layers/fibres may be in the 0xc2x0 main direction, with 20% each at 45xc2x0 and 135xc2x0, and 10% at 90xc2x0.
The invention thus results in a composite article that is contoured, for example to conform with the shaping of a joggle or ramp, without any of the fibres in the said load-bearing direction being cut. The contouring has been effected by applying a force to the planar article whilst simultaneously preventing any buckling thereof.
Preferably, the article is constrained by being loaded into a support that allows flexibility in a direction to achieve the required contouring but which is rigid in a direction perpendicular thereto. Advantageously, the support may comprise an elastomeric material reinforced with spaced apart rigid rods, of aluminium or steel for example.
The support is preferably mounted in a rigid frame that is located around the forming tool and that is arranged to guide the article in a direction towards the contoured surface during the forming process.
The force may be provided by a press acting on an edge of the planar article. Alternatively, the article, tool, frame and support may be located in an enclosure that is then evacuated.
The contoured, planar reinforcing article thus produced, may be subjected to a further forming operation in which heat is again applied thereto and a force is applied to effect further forming thereof. For example the further forming operation may be arranged to produce a fold about a line in the planar surface of the article, the line preferably extending substantially parallel to the direction of the said fibres, thus producing an L-shaped article. A further forming operation may be arranged to produce a channel-shaped article, or one of any desired shape.
Thus, the present invention provides a method of forming a substantially planar, multi-layer composite article such that it is able to conform to a substrate that comprises three contiguous surfaces each of which is angled, i.e. lies at other than 0xc2x0, to the other two surfaces. The surfaces may be generally planar, but may for example, be convoluted and may comprise a ramp or a joggle.
The plastics material of the article may be thermo-plastics. Alternatively, the plastics material may be curable, for example being a thermoset, with curing being effected after the forming thereof has been completed.
The reinforcing fibres may be of carbon, aramid or glass, for example.
It is to be understood that although reference has been made to directions with respect to the orientation of continuous fibres in the plane of the article, the composite article may include fibres in layers at various other angles to this primary load-bearing direction.
The method of the present invention finds particular application to a multi-layer composite article, due to its increased rigidity with respect to a relatively flexible single ply article. Thus, the article to which the present invention is particularly beneficial is envisaged as comprising at least two plies, each of which would typically be between 0.125 and 0.25 mm thick, and typically would have a minimum of 8 plies, thus giving a thickness of 2 to 3 mm, and may even be formed as a 5 mm thick stack of 20 plies, or more. It will be appreciated that the number of plies, and thus the thickness of the article would depend on the loading to which it was to be subject in use.
In accordance with a further aspect of the present invention, there is provided apparatus for forming a substantially planar, multi-layer composite article, comprising means for constraining the article in a direction substantially perpendicular to its plane, means for mounting the article over a contoured surface to which it is to be conformed, and means for applying a force in a direction in the plane of the article to urge the article into conformity with the contoured surface.
The apparatus is preferably arranged to carry out the features of the method of the invention.